Polyimides constitute a class of valuable polymers characterized by high strength, thermal stability, inert character, and high T.sub.g, among other properties. Polyimides find extensive use in electronic applications where they are useful in forming dielectric films in electrical and electronic devices such as motors, capacitors, semiconductors, printed circuit boards and other packaging structures. Typical uses for polyimides include protective and stress buffer coatings for semiconductors, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, bonding layers for multilayer circuits, final passivating coatings on electronic devices and the like.
The increased complexity of the applications for polyimides, especially in the area of electronics, has placed a greater demand on the ability to tailor the properties of the polyimide for a specific application. Microelectronic devices often consist of multilayer structures with alternating layers of conductors (metals or semiconductors) isolated by layers of dielectric insulators (e.g., polyimides). The manufacture of these devices often involves a sequence of high temperature heating and cooling cycles. Stresses can be generated during these processes due to differences between the thermal expansion and moduli of the various layers of materials. These stresses may degrade the performance of the device and/or lead to premature device failure.
As electronic components become smaller, the control of stress becomes an even greater concern. A large part of the stress is related to the difference in the thermal expansion of the components. It is therefore desirable to be able to control the coefficient of thermal expansion (CTE) for dielectric materials so that it can be matched as closely as possible to the CTE's of other materials in the device.
Examples of high and low CTE polyimides have been disclosed in, for example, U.S. Pat. Nos. 4,690,999 and 4,137,933. However, it is frequently desirable to control the CTE more precisely and it is also necessary to take into consideration the other properties of the polyimide, e.g., electrical, mechanical and chemical properties. It therefore would be desirable to have a process for preparing a polyimide film having a preselected value for CTE.